Method for producing powdered metals and alloys

ABSTRACT

METHOD FOR PRODUCING POWDERED METALS AND ALLOYS THAT ARE FREE FROM OXIDATION COMPRISING ATOMIZING MOLTEN METAL WITH A FLUID JET TO FORM DISCRETE PARTICLES THEREOF, DIRECTING SAID ATOMIZED PARTICLES TO A COLLECTING CHAMBER HAVING THEREIN A RESERVOIR OF AN INERT CRYOGENIC LIQUID, SUCH AS LIQUID NITROGEN, DEPOSITING SAID PARTICLES IN SAID LIQUID TO COOL THE PARTICLES AND AT THE SAME TIME PROTECT THEM AGAINST OXIDATION DURING COOLING.

Feb. 29, 1972 v. R. THOMPSON ErAL 3,646,177

METHOD FOR PRODUCING POWDERED METALS AND ALLOYS Filed April 23, 1970 IIVVE'NTOR VERNON R. THOMPS N 8 ROBERT W O/EHL A Homey United States Patent O 3,646,177 METHOD FOR PRODUCING POWDERED METALS AND ALLOYS Vernon R. Thompson, Pittsburgh, Robert W. Diehl,

Robinson Township, Allegheny County, Pa., assignors to Crucible Inc., Pittsburgh, Pa.

Filed Apr. 23, 1970, Ser. No. 31,105 Int. Cl. B22d 23/08; B29c 6/00, 23/00 US. Cl. 264-12 4 Claims ABSTRACT OF THE DISCLOSURE It is known in the art of powder metallurgy to produce metal powders for subsequent compacting by striking a stream of molten metal with a jet of inert gas, such as nitrogen, whereby the molten metal stream is atomized to form particles. These particles are quenched to room temperature for subsequent handling and use. In view of the increased surface area of the molten metal upon atomization, prior to cooling to room temperature the metal or alloy is highly susceptible to extreme surface oxidation. This susceptibility to oxidation is further enhanced should the alloy have constituent metals that are of the highly oxidizable types, such as titanium and zirconium. The molten metal particles produced upon atomization must be quickly cooled to room temperature to prevent them from sintering or agglomerating, which renders them unsuitable for subsequent use in compacting into articles by conventional powder-metallurgy techniques. Cooling that is sufficiently rapid for this purpose can only be achieved conventionally by using a water quench; however, contact with water causes rapid and severe oxidation of the particles. When the metal or alloy is oxidized during cooling to room temperature, this contamination will cause any powder-metallurgy products produced from the powder to have a correspondingly high oxygen content, which characteristically results in poor product quality, particularly from the standpoint of cleanlines Consequently, it has heretofore been necessary to subject metal and alloy powders that have been water quenched to a cleaning operation prior to compacting by powder-metallurgy techniques. Such cleaning operations are quite cumbersome and time consuming and therefore add significantly to the processing costs and thus the cost of the final product.

It is accordingly the primary object of the present invention to provide a method for producing metal and alloy powders wherein the powders are cooled to room temperature without any appreciable oxidation, and therefore subsequent cleaning operations prior to compacting are avoided.

Another more specific object of the invention is to provide for the quenching or cooling of molten atomized particles of molten metal or alloy by the use of an inert cryogenic liquid, such as liquid nitrogen, argon or helium, whereby the particles are rapidly cooled to room temperature and at the same time protected against surface oxidation.

These and other objects of the invention, as well as a complete understanding thereof, may be obtained from 3,646,177 Patented Feb. 29, 1972 Too the following description, specific examples and drawing, in which:

The single figure thereof is a vertical section of apparatus suitable for the practice of the method of the invention.

Broadly, in the practice of the invention a quantity of molten metal or alloy is formed into a stream that is contacted by a fluid jet to atomize the same and thereby form discrete particles. The particles are directed to a collecting chamber having a reservoir of an inert cryogenic liquid, which is preferably nitrogen, but argon and helium may also be used The particles are deposited in the liquid to cool the same, and at the same time they are protected by the inert liquid against oxidation. The fluid jet preferably is an inert gas and the chamber is preferably provided with a nonoxidizing atmosphere, either by evacuation to a subatmospheric pressure or by purging with an inert gas preferably of the cryogenic liquid of the reservoir. After atomizing has been completed the inert cryogenic liquid is drained from the chamber, the chamber is opened to the atmosphere and the remainder of the liquid is removed from the powder by volatilization. With reference to the single figure of the drawing there is shown one embodiment of apparatus, designated generally as 10, adapted for the practice of the method of the invention The apparatus 10 consists of a closed chamber 12 provided with a reservoir 14 of an inert cryogenic liquid, such as for example liquid nitrogen. Adjacent the base of the chamber 12 is an outlet or drain 16 with an associated valve 18, which when opened will permit drawing of the cryogenic liquid from the chamber. Mounted over the open top of the chamber 12 is a tundish 20 having a central bottom nozzle opening 22. A quantity of molten metal 24 for atomization is poured into the tundish from a ladle or other suitable transport device, not shown. The tundish 20 is in the customary manner lined with a suitable refractory material not shown. With the tundish 20 mounted atop the chamber 12, which is the position shown in the drawing, the chamber 12 is sealed against the atmosphere by O-ring seal 26. In this manner, if the chamber interior is purged by the use of an inert gas or pumped to a subatmospheric pressure, the resulting nonoxidizing atmophere therein may be preserved during subsequent atomization of the molten metal from the tundish. Beneath the top of chamber 12 and extending through the chamber walls and into the interior thereof is a plurality of gas jets 28. The jets 28 are directed toward the axis of the nozzle 22 of the tundish a short distance beneath the end thereof.

In the practice of the invention the tundish 20 is filled with molten metal 24 which emerges in a stream 30 from nozzle 22. An inert gas 32 from gas jets 28 strike the molten metal stream 30 and atomize it into molten metal particles 34. The particles 34 drop to the bottom of the chamber and into the reservoir 14. The cryogenic liquid of the reservoir quickly cools the particles 34 before they can agglomerate and also during cooling protects them against oxidation. Since a non-oxidizing atmosphere is provided within the chamber, the particles are also protected against oxidation during their passage from the tundish nozzle 22 to the reservoir 14. When all the metal in the tundish has been atomized the tundish is removed from the chamber 12 to open it to the atmosphere. At this time valve 18 of drain 16 is opened to drain the cryogenic liquid from the chamber. Any liquid remaining on the particles is subsequently removed by volatilization when the interior of chamber 12 is open to the atmosphere. The cooled powder is then removed from the chamber for subsequent storage and use in compacting of powder-metallurgy products.

The term inert as used herein means that the particular cryogenic liquid being used will not have a deleterious efiect on the particular metal or alloy being quenched therein.

As a specific example of the practice of the invention a tool steel (REX 71) and a nickel-base superalloy (Astroloy) of the following compositions were employed:

REX 71:

C 1.20 Mn 0.30 S 0.02

P 0.02 Si 0.30 Cr 4.10 V 1.25 W 10.0 Mo 5.2 Co 12.2 Fe Bal.

Astroloy:

C 0.10 Al 4.35

Ti 3.35 Mo 5.25 Cr 15.0 C 18.5 Fe max 1.0 B 0.030

Ni Bal.

These alloys were subjected to the following conventional practice and a similar quantity of the alloys was subjected to the practice of the invention as hereinafter described.

Separate charges of the above-listed two alloys were heated in an induction melting crucible to molten form, poured into a tundish and permitted to exit through a refractory nozzle in the tundish bottom and into an atomizing chamber. The apparatus employed was similar to that shown in the figure and described herein. The atomizing chamber had been purged with gaseous nitrogen. The molten metal exiting from the tundish nozzle was contacted with a stream of gaseous nitrogen at a velocity suificient to atomize the metal and thus produce molten metal particles. The bottom portion of the atom'izing chamber was filled with water, in the case of the conventional practice, and with liquid nitrogen, in

the case of the quenching practice in accordance with the Oxygen content (parts by million) Quenching medium REX 71 Astroloy Water 500 400 Liquid nitrogen It may be seen that the particles quenched in the liquid nitrogen had much lower oxygen contents than the particles quenched in water.

We claim:

1. A method for producing powdered metal and alloys characterized by freedom from oxidation comprising atomizing molten metal with an inert gas jet to form discrete particles thereof, directing said atomized metal particles to a collecting chamber having therein a reservoir of an inert cryogenic liquid, depositing said particles in said liquid to cool said particles, whereby said particles are protected against oxidation by said inert cryogenic liquid during cooling.

2. The method of claim 1 wherein said inert cryogenic liquid is a liquid selected from the group consisting of nitrogen, argon and helium.

3. The method of claim 2 wherein said chamber is provided with a nonoxidizing atmosphere.

4. The method of claim 3 wherein said chamber is at a subatmospherie pressure during the depositing of said particles therein.

- References Cited UNITED STATES PATENTS 2,630,623 3/1953 Chisholm et al. 264-l2 2,967,351 1/1961 Roberts et a1 264l2 3,516,879 6/1970 Paine 264-28 ROBERT F. WHITE, Primary Examiner J. R. HALL, Assistant Examiner US. Cl. X.R. 264-28 

